Residual current devices

ABSTRACT

In one aspect, the invention provides a residual current device (RCD) for protecting a circuit by tripping in response to an imbalance signal representative of residual current imbalance in the circuit. The RCD trips the circuit when the imbalance signal exceeds a predetermined threshold rating. The RCD comprises test means for increasing the imbalance signal so as to test operation of the RCD against the rating. In another aspect the RCD comprises: a current transformer for generating an imbalance sense current in a sense coil in response to a current imbalance in an electrical supply; and a degaussing coil for substantially removing remanence in the current transformer by application of a degaussing signal to the degaussing coil.

The present invention relates to residual current devices (RCDs). Morespecifically, it relates to RCDs that have a test facility which, whenactuated, causes the device to trip.

RCDs are installed for protection against certain potentially dangeroussituations arising in electrical supply installations. As shown in FIG.1, an electrical supply installation 10 has a number of conductors 11(typically neutral and live conductors for single phase A.C. suppliesand three live conductors or three live and one neutral conductor forthree phase A.C. supplies). The conductors 11 connect to a load circuit12 (e.g. a domestic ring main to which appliances are connected). Aknown RCD 13 operates by disconnecting the supply from the load circuit12 when an imbalance is detected in the current flowing in theconductors 11. This imbalance is due to current flowing to earthindicating, for example, poor insulation or electrocution of a person.

The RCD 13 has a current transformer 4 consisting of a toroidal magneticcore surrounding the conductors 11. A sensor coil (not shown) is woundaround the core so that any imbalance in the current flowing in theconductors 11 causes a sensor signal current 5 to be induced in thesensor coil, which current is proportional to the current imbalance. Anelectronic signal processing circuit 6 analyses the sensor signalcurrent 5 to determine if the current imbalance is at or above a pre-settrip threshold indicative of a potentially dangerous condition in thesupply circuit. The device then trips the circuit by providing power toan actuator 17 to actuate a switch 18 to isolate the supply from theload circuit 12.

RCD devices are required to be fitted with a test button. Pressing thebutton causes the device to trip, which allows a person to testsatisfactory operation of the device. Activation of the test buttoncloses a contact causing a test circuit to introduce a signal tosimulate a residual current so that the whole signal path from thesensor to the switch is included in the test. This may be achieved bythe circuit shown in FIG. 2. Some of the current in one of theconductors 21 a of live and neutral supply conductors 21 a, 21 b flowsvia a resistor 22 so as to bypass the current transformer 4 when a testbutton is pressed to close a contact 24. There are many disadvantageswith this approach. Firstly, connection of the test circuit to the mainsconductors 21 a, 21 b is required, which can be mechanically awkwardwithin RCD devices. Secondly, the apparent residual current produced isvoltage dependent and also dependent on the tolerance and stability ofthe resistor 22. In practice, currents much greater than the tripthreshold are induced so as to ensure tripping (typically two and a halftimes, and in some cases as much as five times, the rated trip value).This tests that the device will operate, but not that it willnecessarily operate at the rated trip value. Thirdly, no account istaken of any standing residual current already in the circuit. In thetest, the device simply adds the test residual current to any standingresidual current already present. Again this means that the test is notcarried out at the rated trip value.

The following further problems may also arise. If the device fails totrip for any reason when the button is pressed, and the button is helddown, the resistor 22 can quickly become very hot and burn. The devicemay be subjected to voltage variations in the supply. As well asaffecting the accuracy of the test, high voltage pulses that may occurbetween the live and neutral conductors 21 a, 21 b can give rise toarcing at the contact 24. RCDs are made with different trip thresholdratings and so the resistor 22 must be changed to suit the threshold,which is inconvenient for production.

Another known method of implementing the test function is shown in FIG.3. A magnetic field is introduced into a core 33 of the currenttransformer 4. A second winding 31 is provided on the transformer core33. The winding is placed in series with a resistor 35 in a test circuitbetween the live conductor 21 a and neutral conductor 21 b. When thetest button 23 is pressed a contact 34 closes the circuit and a testsignal current flows through the second winding 31. This will induce acurrent in the sense coil 32. Typically the test signal current is muchsmaller than the sensor signal current required to trip the device dueto current gain in the transformer 4. A 100 turn winding means only1/100^(th) of the trip threshold current is required to produce anapparent residual current sufficient to cause a trip. This methodreduces the problem of resistor heating, but does not overcome most ofthe disadvantages of the previous method, such as supply voltageconnection, inaccuracy due to standing residual current, high voltagecontact rating and resistor tolerance and stability.

Another problem associated with current transformers is that ofremanence. This is an effect where the magnetic material forming thecore of the transformer becomes magnetized. This effectively lowers itspermeability and prevents it from conveying further magnetic flux. Thecoupling effect of the transformer is then effectively lost or reducedand the device becomes insensitive. Magnetisation can occur when heavyfault currents flow and are switched off when at peak value by thetripping mechanism leaving remanent magnetisation. When this hasoccurred and the device is subsequently reset, insensitivity due toremanence means that the device may be reset when a fault is stillpresent in the supply circuit.

It is an aim of the present invention to provide an RCD whichsubstantially alleviates these problems.

According to a first aspect of the present invention there is provided aresidual current device (RCD) intended for tripping an electrical supplyfrom a circuit to be protected when a residual current imbalance in thecircuit exceeds a predetermined threshold rating, the RCD comprising:

sense means for generating an imbalance signal representative ofresidual current imbalance in the circuit;

trip means intended for tripping the residual current device when theimbalance signal exceeds the predetermined threshold rating so as todisconnect the electrical supply from the circuit; and

test means for increasing the imbalance signal to a level whichsubstantially corresponds to the predetermined threshold rating wherebya trip at said rating indicates a successful test.

It is an advantage that the device may be tested for whether or not theRCD trips at or near the rated value. That is, a successful testindicates that the device is operative to trip at the intended thresholdrating. An unsuccessful test is one where the device trips when theimbalance signal is below or above the threshold, this conditionindicating that the device is not operating at its rating. The test istherefore more rigorous and accurate than the test provided in prior artdevices.

The sense means may be operative for measuring an amount of any residualcurrent imbalance in the circuit.

The test means may be operative for calculating a difference valuecorresponding to the difference between the measured residual currentimbalance and the predetermined threshold rating. The difference valuemay be applied such that the increase in the imbalance signal issubstantially instantaneous. Alternatively, the testing means may beoperative to ramp up or progressively increase the imbalance signal froma low or zero value to the predetermined threshold value. Thisalternative provides for determining the level of current imbalance atwhichever level the device trips. This advantageously provides fortesting whether the device trips at a level which is less than thepredetermined threshold.

In embodiments of the invention, the test means effectively introduces asimulation residual current imbalance into the device so that the sensemeans senses the sum of any residual current imbalance in the circuitbeing protected and the simulated residual current.

In a preferred embodiment, the sensor means comprises a currenttransformer having a sense coil, the imbalance signal being an imbalancesense current induced in the sense coil. The means for increasing theimbalance signal may include a test coil, wherein a test current appliedto the test coil is operable for introducing the simulation currentimbalance in the form of a magnetic field in the transformer, therebyinducing the increase in the imbalance sense current in the sense coil.

The testing means may be coupled to a processor that monitors theimbalance signal and determines the simulation current imbalancerequired to increase the imbalance signal to a level that corresponds tothe rated value. It is an advantage that, if the processor detects acurrent imbalance below the rated trip value (a standing currentimbalance), then it determines how much to increase the imbalance signalto reach the level that corresponds to the rated trip value, and therebyprovides a more accurate test than the prior art devices.

The processor may include an analogue to digital converter (ADC) forconverting the current imbalance signal to a digital form, amicro-controller unit (MCU) for processing the digital signal and forproviding a digital output signal, and a digital to analogue converter(DAC) for converting the digital output signal to an analogue testsignal. The digital processing enables the generation of a test currenthaving a waveform and phase profile appropriate for providing therequired sum.

An advantage of synthesising a waveform for the simulation currentimbalance directly from the processor is that it is independent of theelectrical supply and any variations therein. A further advantage isthat the waveform can be synthesised by the processor based on thestanding residual current determined from the imbalance signal. Thismeans that whatever waveform, phase angle or frequency the standingresidual current has, the processor can synthesise a simulation currentimbalance waveform, which, when added to the standing residual currentwaveform, ensures that the device is tested against the rated value.

Preferably, the processor is an integrated circuit in the RCD. Anintegrated circuit is an effective, low cost, space-efficient processor,which is simple to assemble into an RCD.

According to a second aspect of the present invention there is provideda residual current device (RCD) comprising:

a current transformer for generating an imbalance sense current in asense coil in response to a current imbalance in an electrical supply;and

a degaussing coil for substantially removing remanence in the currenttransformer by application of a degaussing signal to the degaussingcoil.

The degaussing coil may be combined with a test coil forming part of atesting means in a device according to the first aspect of the presentinvention as defined above.

Degaussing is a method of removing a remanent magnetic field by drivingthe transformer core with an alternating field which decreases inamplitude over several cycles. Removing remanence means that a device,which has been desensitised due to a remenant magnetic field in thetransformer core, can be resensitised and thereby re-establish thedevice's sensitivity so that it will continue to function in therequired manner. By degaussing to remove remanence, the device can bere-set after a trip while ensuring that the device will trip againwithin a very short time if the circuit still has a fault.

The degaussing signal may be applied to the degaussing coil under thecontrol of a processor. The processor may be configured to apply thedecaying alternating field at a high frequency so that the degaussingsignal is not detectable by the RCD's residual current detection system.This ensures that degaussing is achieved in a very short time and thatremanence is removed quickly when re-setting the RCD. The RCD must becapable of tripping within a specified number of cycles of the A.C.supply and so the high frequency degaussing signal ensures thatremanence is removed in fewer than the specified number of cycles. Thehigh frequency degaussing also enables the processor to be configured tocontrol degaussing during normal operation.

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic circuit diagram of a known electrical installationhaving a known RCD, as hereinbefore described;

FIG. 2 is a schematic circuit diagram of a known test circuit for anRCD, as hereinbefore described;

FIG. 3 is a schematic circuit diagram of another known test circuit foran RCD, as hereinbefore described;

FIG. 4 is a schematic circuit diagram of a test and degaussing circuitfor an RCD in accordance with the invention;

FIGS. 5, 6 and 7 are graphs showing current waveforms as may be found inan RCD according to the invention; and

FIG. 8 is a graph showing a degaussing current waveform, for use in anRCD according to the invention.

Referring to FIG. 4 a power supply installation has a live conductor 40and a neutral conductor 42, for supplying current from a supply to aload circuit 44. An RCD 46 includes a toroidal transformer 48 having acore 50 which surrounds the live and neutral conductors 40, 42. A sensecoil 52 and a test coil 54 are wound on the core 50. A current inducedin the sense coil 52 is supplied as an input to an electronic processor56. A switch mechanism 58, actuated by an actuator 60 under the controlof the processor 56, breaks the live and neutral conductors 40, 42 whena predetermined level of residual current is detected.

In the electronic processor 56 the input current from the sense coil 52flows to a transresistance amplifier 58, having a voltage output that islinearly related to the input current. The output voltage of thetransresistance amplifier 58 is then fed via a lowpass filter 60 (toprevent aliasing) to an analogue-to-digital converter (ADC) 62 whichoutputs the voltage as a digital electronic signal. The digital signalis fed to a micro-controller unit (MCU) 64 via a digital bus 66. The MCU64 has an output 68 for controlling operation of the switch actuator 60.

The RCD 46 is provided with a test button 70 for closing a contact 72 toinitiate a test under the control of the MCU 64. A digital test signalprovided by the MCU 64 is fed via the bus 66 to a digital-to-analogueconverter (DAC) 76, which outputs an analogue test current to the testcoil 54.

In use a current imbalance between the live and neutral conductors 40,42, generates a magnetic field which induces a sense current in thesense coil 52. The sense current is amplified by the transresistanceamplifier 58 and converted into a digital signal by the ADC 62 and readby the MCU 64. If the MCU 64 determines that the current imbalance isabove the predetermined rated trip value, then a trip signal is appliedto the MCU output 68 such that the switch acuator 60 actuates the switch58 to break the live and neutral conductors 40, 42, and therebyinterrupt electrical supply to the load circuit 44.

The device may be tested while operational in an untripped condition.Pressing the test button 70 closes the contact 72 and initiates thetest. The MCU 64 determines the level of the current imbalance beingsensed by the sense coil 52, and calculates the amount by which thecurrent from the sense coil 52 must be increased for the RCD 46 to tripat its rated trip value. The calculated increase is provided by means ofthe test coil 54. A test current is provided to the test coil, whichgenerates a magnetic field in the core 50 of the transformer 48. Themagnetic field generated induces an increase in the sense current in thesense coil 52. The MCU calculates the test current required to testwhether or not the RCD trips at the rated value.

The sense coil 52 is typically 1000 turns of wire and the test coil 54is typically 100 turns. The current in the sense coil 52 is linearlyrelated to the residual current by a factor determined by the turnsratio between the electrical circuit conductors 40, 41 (the primary coilof the transformer) and the sense coil 52. Therefore, a 10 mA RMSresidual current induces a 10 micro-amp RMS current in the sense coil 52for the 1:1000 turns ratio. A working bandwidth from 20 Hz to 2 kHz isreadily achievable and adequate for RCD purposes. The transresistanceamplifier 58 is characterized by having low (almost zero) inputimpedance which is necessary to ensure the sense current is directlyrelated to the residual current by a fixed 1:1000 ratio over the workingbandwidth. The output of such an amplifier is a voltage linearly relatedto the input current with a typical gain of 10000V/A.

The ADC 62 periodically samples the voltage and each time outputs adigital electronic value of typically 10 bits. The ADC 62 can be timemultiplexed so as to also sample the line voltage of the supply via apotential divider network 74 allowing mains frequency to be monitored.The processor 56 measures the frequency of the residual current waveformand the sample frequency is adjusted such that a fixed number of samplesper cycle are taken. A rate of 64 samples per cycle of the residual at50 Hz gives a sample rate of 3200 Hz, whereas at 60 Hz the sample rateis 3840 Hz. An algorithm executed on the MCU 64 determines the frequencyof the residual current, but in cases where it cannot be determined(e.g. the amplitude is zero, or the signal is random, or the signal isoutside the expected range of values) then the line voltage frequencycan be measured and used.

With the residual current waveform accurately represented by digitalvalues, it is possible to apply digital signal processing techniques todetermine various parameters of the signal and in particular tocalculate its RMS value to cause a trip if this exceeds the setthreshold rating. The digital processing is performed by the MCU 64,which includes control circuitry, arithmetic circuitry, a read/writememory for storage of variable values and a non-volatile read-onlymemory which stores an executable software program for the whole MCU 64to follow. Other peripheral devices not shown are also present includingpower supplies, clock circuits and power-on reset circuits.

The calculation of the residual current RMS is performed over a wholenumber of cycles to ensure accuracy. Ten cycles of the residual waveformis a sufficient period to perform the calculation and since the samplefrequency is adjusted to give a fixed number of samples per cycle (say64) then the total calculation requires 640 samples. For a 50 Hzresidual current frequency this therefore takes 200 mS to process 640samples and at 60 Hz takes 167 mS. In both cases tripping occurs withinthe time set by published standards. The software is written into theMCU 64 at manufacture using a non-volatile memory. The non-volatilememory also contains associated configuration data, such as the trippingcurrent threshold and calibration data derived from measurements takenat manufacture.

The DAC 76 either directly outputs current or otherwise outputs voltagewhich can be converted to current by a linear current-to-voltageamplifier (transconductance amplifier) or more simply using a fixedresistor. The waveshape and amplitude of the current signal produced bythis system is controlled by the MCU 64 under software control.

Most prior art devices drive a current of up to 2.5 times the trippingthreshold of the device using mains voltage to source a sinusoidalsignal at 50 or 60 Hz. This ensures that whatever standing residualcurrent may already be present, the test current will swamp it andguarantee the device trips. This is effective in causing a trip but doesnot really test the accuracy of the system. By driving a synthesizedwaveform into the test coil 54, the test current is independent ofsupply voltage and does not require a high voltage switch since the testcircuit is connected to a low voltage MCU input.

However, in order for the test coil 52 to induce the correct RMS currentin the sense coil 52 to produce a trip, it is necessary to determine thewaveform of any standing residual current. Standing residual currentsare usually caused by poor insulation or capacitive suppressor networksoften found on motors. The waveform will often be a sinewave in phasewith the mains voltage but it is possible that it could be up to 90degrees out of phase if leakage is purely reactive and maybe up to 180degrees if generating equipment is present in the load circuit. Also,non-sinusoidal residual current waveforms are common but will almostalways be repetitive at the mains frequency. To illustrate this,consider a standing residual current as measured by the processor 56 tobe 20 mA RMS, then the extra apparent residual current to be induced bythe test circuit can be calculated using the following equation:—$\begin{matrix}{x = \sqrt{I_{n}^{2} - s^{2}}} & \left( {{Equation}\quad 1} \right)\end{matrix}$where s is the measured standing RMS residual current, I_(n) is the RMStrip threshold and x is the required extra apparent residual RMS to beinduced by the test circuit such that the resultant measured is equal toI_(n). For a device where the threshold I_(n) is 30 mA then it isnecessary to drive the test coil to produce an extra 22.4 mA RMSmeasured residual to cause tripping. However, the equation above (whichis based on the fact that the resultant RMS of two summed signals isequal to the root of the summed squares of the individual RMS values)assumes the following conditions

-   -   a) that the standing residual and test current are of differing        frequencies    -   b) that the resultant RMS of the sum of the two signals is        calculated over a long period to achieve an accurate result.        Condition “a” can be illustrated by FIG. 5 where two sinusoids        of equal frequency and phase are summed, one being of a peak        amplitude of 1 unit (0.7 units RMS) and the other of 2 units        (1.4 RMS). The resultant according to the equation above is 1.6        RMS or 2.2 peak. However, it is clear in FIG. 5 that the        resultant is of peak amplitude 3.0 and so its RMS value is 2.1.        The equation actually only holds true if the two signals are 90°        out of phase as shown in FIG. 6. It would be possible to measure        the phase of a standing residual current and add the test        current at an appropriate phase to generate the required        resultant but this adds considerable complexity and does not        work with all wave shapes. It is therefore evident that the RMS        calculation has a dependency on the phase between the two        signals being summed and an accurate result is only obtained if        the RMS is averaged over all possible phase differences.

A simpler solution is to adhere to condition “a” and drive the testsignal at a different frequency to any standing residual current. Asdescribed above, the MCU 64 is capable of measuring the frequency, or insome circumstances it is assumed to be the same as the measured supplyfrequency. The test coil 54 can then be driven at a frequency 20% higheror lower than the measured residual current frequency (e.g. 40 Hz if themeasured frequency is 50 Hz). The resultant is shown in FIG. 8. The RMSof the resultant is found to be correct as predicted by Equation 1above, and will in fact work for any wave shape of standing residualcurrent. It is also true that any wave shape for the test signal canalso be used and the use of a square wave test signal rather than asinusoid can be simpler to synthesise. Another way of looking at this isthat the use of different frequencies means that the dependency of theresultant RMS on phase is lost because the two signals are added overtime at all combinations of phase.

Condition “b” above, requires measurement of the resultant of thestanding and induced test current signals to be performed over a greatlength of time to achieve accuracy. The tripping time at the ratedthreshold for most RCDs is set at 300 ms maximum by the relevantstandards. Therefore, when the test button 70 is pressed the device hasabout 14 mains cycles (280 mS) to initiate the trip. This number ofcycles does give reasonable accuracy but improved accuracy and trippingtime can be achieved with some care. With reference to FIG. 7 it isevident that a beat frequency is present equal to the difference in thefrequencies of standing residual current and test current, in this case10 Hz for a 50 Hz residual current Over the ten-cycle period shown (200mS at 50 Hz) two beats are present and it is notable that the relativephases of the three traces are the same at the start and end of theperiod shown. The result is accurate since all combinations of possiblephase between the two signals have been used in the calculation exactlytwice, meaning any initial phase is irrelevant and phase dependency islost. A measurement period which is not a multiple of the beat periodgives less accurate results since some phase combinations are seen moretimes than others and so initial phase becomes a factor in thecalculated RMS of the resultant. The test signal is calculated as afixed percentage of the standing residual current frequency such thatover the period where a fixed number of samples are used to calculatethe resultant RMS there will be an integer multiple of cycles of thebeat frequency produced between the standing residual current and testsignal frequencies.

The test current calculation must take into account the turns ratio ofthe sense and test coils so that the induced current ratio is correct,as well as the wave shape used for the test signal. Also, initialtolerances in the system can be accounted for using calibration valuesstored in memory at manufacture to modify the test current amplitude.Once the residual current frequency has been determined in the mannerdescribed above, then on initiation of a test by operation of the testbutton 70 a test signal of the calculated amplitude is driven into thetest coil 54 at a frequency different to that of the residual current54. The measurement system will be operating normally by continuouslymeasuring the apparent RMS values detected in the sense coil 52 over afixed number of mains voltage cycles and causing a trip when necessary.

Another feature of the device is the ability to effectively counter theproblem of remanence described above. To counter this problem theremanent magnetic field in the transformer core 50 can be removed bydriving the core 50 with an alternating field which decreases inmagnitude over several cycles. This technique is called degaussing. Sucha signal can be driven into the test coil 54 to permit degaussing undersoftware control. It is particularly useful to perform degaussing atstartup of the device as this is when the core 50 may have been leftmagnetized following a fault which caused a trip. However, periodicdegaussing can be implemented during normal operation if desired,providing it can be done quickly without effecting normal operation ofthe device. If the degaussing signal frequency is much higher than theoperating band to which the residual current sensing circuit issensitive, then the high frequency degaussing signal will not be seendirectly by the measurement system. A suitable type of waveform is shownin FIG. 8. It consists of a decaying waveform whose initial amplitude issufficiently high to cause magnetic saturation of the core (i.e. itcannot become more strongly magnetized). The wave form has a peakamplitude of around 2A-turns of the test coil 54, so for a 100 turn testcoil 54 this means a current of 20 mA peak is required. The subsequentdecaying waveform leaves the core less and less magnetized after eachcycle. A high frequency waveform of around 10 is suitable and a decayrate of 80% per millisecond over a two-millisecond period achievesdegaussing in a short time. However, the optimum parameters of thewaveform are greatly dependent upon the dimensions and material of thetoroidal core. There are no extra components required to performdegaussing as the proposed components of the test circuit of FIG. 4 areable to produce the required signal. The synthesis of the waveform isundertaken by the MCU 64 under software control. The waveshape used neednot be sinusoidal as suggested, other shapes such as rectangularwaveforms are equally effective and are simpler to synthesize.

1. A residual current device (RCD) for protecting a circuit by trippingin response to an imbalance signal representative of residual currentimbalance in the circuit, the RCD tripping the circuit when theimbalance signal exceeds a predetermined threshold rating, wherein theRCD comprises test means for increasing the imbalance signal so as totest operation of the RCD against the rating.
 2. The device of claim 1,further comprising sense means for generating said imbalance signal. 3.The device of claim 2, wherein the sense means is operative formeasuring an amount of any residual current imbalance in the circuit. 4.The device of claim 3, wherein the test means is operative forcalculating a difference value corresponding to the difference betweenthe measured residual current imbalance and the predetermined thresholdrating.
 5. The device of claim 4, wherein the difference value isapplied such that the increase in the imbalance signal is substantiallyinstantaneous.
 6. The device of claim 4, wherein the test means isoperative to ramp up or progressively increase the imbalance signal froma low or zero value to the predetermined threshold value.
 7. The deviceof claim 2, wherein the sense means comprises a current transformerhaving a sense coil, the imbalance signal being an imbalance sensecurrent induced in the sense coil.
 8. The device of claim 2, wherein thetest means introduces a simulation residual current imbalance into thedevice so that the sense means senses the sum of any residual currentimbalance in the circuit being protected and the simulation residualcurrent.
 9. The device of claim 8, wherein the means for increasing theimbalance signal includes a test coil, wherein a test current applied tothe test coil is operable for introducing the simulation currentimbalance in the form of a magnetic field in the transformer, therebyinducing the increase in the imbalance sense current in the sense coil.10. The device of claim 8, wherein the testing means is coupled to aprocessor that monitors the imbalance signal and determines thesimulation current imbalance required to increase the imbalance signalto a level that corresponds to the rated value.
 11. The device of claim10, wherein the processor includes an analogue to digital converter(ADC) for converting the current imbalance signal to a digital form, amicro-controller unit (MCU) for processing the digital signal and forproviding a digital output signal, and a digital to analogue converter(DAC) for converting the digital output signal to an analogue testsignal.
 12. The device of claim 11, wherein the processor is operablefor generation of a test current having a waveform and phase profileappropriate for providing the required sum.
 13. The device of claim 10,wherein the processor is an integrated circuit in the RCD.
 14. Aresidual current device (RCD) comprising: a current transformer forgenerating an imbalance sense current in a sense coil in response to acurrent imbalance in an electrical supply; and a degaussing coil forsubstantially removing remanence in the current transformer byapplication of a degaussing signal to the degaussing coil.
 15. Aresidual current device (RCD) for protecting a circuit by tripping inresponse to an imbalance signal representative of residual currentimbalance in the circuit, the RCD tripping the circuit when theimbalance signal exceeds a predetermined threshold rating, said RCDcomprising: a current transformer for generating an imbalance sensecurrent in a sense coil in response to a current imbalance in anelectrical supply; and test means for increasing the imbalance signal soas to test operation of the RCD against the rating, said test meansincluding a test coil combined with a degaussing coil for substantiallyremoving remanence in the current transformer by application of adegaussing signal to the degaussing coil.
 16. The device of claim 15,wherein the degaussing signal is applied to the degaussing coil underthe control of a processor.
 17. The device of claim 16 wherein theprocessor is configured to apply the degaussing signal so as to drivethe transformer core with an alternating magnetic field which decreasesin amplitude over several cycles.
 18. The device of claim 17, whereinthe processor is configured to apply a decaying alternating field at ahigh frequency so that the degaussing signal is not detectable by theRCD's residual current detection system.
 19. The device of claim 17 orclaim 18, wherein the degaussing signal has a sinusoidal or arectangular waveform.